1. Field of the Invention
The present invention relates to an electronically controlled automatic transmission system mounted on a vehicle.
2. Description of the Related Art
In a typical conventional automatic transmission system, clutches and brakes provided in the automatic transmission are activated by changing the level of oil pressure in an oil-hydraulic circuit to shift gears. To effect gear-shifting, a clutch or brake which has been engaged is released and a desired clutch or brake is engaged instead. Accordingly, a temporary neutral state may occur during a gear-shifting operation. If such a temporary neutral state occurs during acceleration of the vehicle, it is likely that the engine speed will suddenly increase or the torque which has been transmitted will suddenly be cut off, thus causing a shock to the vehicle. To reduce this shock, there has been proposed a method wherein, before a clutch or brake which is to be released is completely released, a clutch or brake which is to be engaged is gradually engaged.
This conventional method causes temporary double engagement in the course of a gear-shifting operation. If the double engagement is heavy, a shock is generated, whereas, if the double engagement is light, the transmitted torque is suddenly cut off, thus causing a shock to the vehicle, as stated above. The condition of double engagement is affected by variations in the oil pressure and mechanical errors. It is therefore impossible with the prior art method to completely eliminate the fear of a shock occurring during a gear-shifting operation.